Apparatus for focusing high-energy particles



I N VEN TORS.

W, K. H. PANOFSKY ET AL Filed March l5, 1950 Kuh.

WOLFGANG /f H. PA NoFs/o/ APPARATUS FOR FOCUSING HIGH-ENERGY PARTICLES Nov. 13, 1951 Patented Nov. 13, `1951 APPARATUS FOR FOCUSING HIGH-ENERGY PARTICLES Wolfgang K. H. Pancfsky and William R. Baker, Alameda, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Y Application March 15, 1950, Serial No. 149,812

Y 6 Claims. i '.This invention relates to a magnetic arc lens and more particularly to an apparatus for focusing high energy particles utilizing the magnetic field` surrounding an electric arc discharge.

ln recent years great strides have been made in developing accelerators for chargedl particles such as cyclotrons, betatrons, synchrotrons, linear accelerators, and Van de Graaif genera-tors. Such known accelerators have inherent characteristics which prevent the production of beams of high energy particles having a high intensity over a small cross-sectional area. Since a majority of the work which it is desired to accomplish with accelerators of the above-mentioned type require beam density rather than total current, a focusing device is desirable for use therewith.

It has been found that the desired focusing of a beam of high energy particles is possible by utilizing magnets along the path of the beam; however, the use of such magnets, while practical in principle, involves the use of magnets having a large physical size and of severe modiiications in the structure of the accelerator. The present invention overcomes such dimculties by providing a simple current carrying cylindrical conductor which is transparent to the passage of a beam and which is disposed with its axis parallel to the beam. The magnetic eld set-up by the current owing through the conductor establishes a -focusing force upon the particles of the beam. It will be readily apparent that such a device may be easily installed with minor modifications to the accelerator itself.

It is therefore an object of the present invention to provide a new and improved method and apparatus for focusing high energy particles in a beam.

Another object of the present invention is to provide a magnetic arc lens for focusing high energy particles in a beam.

Still another object of the present invention is to provide a current carrying conductor which is cylindrical in form and transparent to a high energy particle beam to accomplish focusing thereof.

A further object of the present invention is to provide a focusing device utilizing the magnetic iield of a cylindrical current carrying conductor to focus a beam of high energy particles.

Another important object of the present invention is to provide an arc discharge between electrodes which are substantially transparent to `a particle beam whereby the arc discharge establishes a magnetic field having a focusing effect upon -such beam.

An additional object of the present invention is to provide an arc discharge between electrodes which are substantially transparent to a high energy particle beam and an electrical circuit for establishing the arc discharge.

Still another object of the present invention is to provide a magnetic arc lens which is readily adaptable for use with particle accelerators and which requires a minimum of modification to install.

Further objects and advantages will be apparent in the following description and claims considered together with the accompanying drawing in which:

Figure 1 is a diagrammatic representation of a cylindrical current carrying conductor in section;

Fig. 2 is a diagrammatic end view of the cylindrical current carrying conductor of Fig. l; and

Fig. 3 is a longitudinal section of the present invention and includes a schematic wiring diagram of a preferred form of electrical connections therefor.

Referring to the drawing in detail, and to Figs. 1 and 2 in particular, a hollow cylindrical current carrying conductor Il having a length L and a.- diameter 2a is disposed in the path of and parallel to a high energy particle beam l2. Flowing through the conductor Il is a uniform arc discharge I3 which establishes a magnetic field as indicated by concentric circles I4 in Fig. 2. With suchV conditions existing the radial variation of the magnetic eld will be found as represented by the curve I6 and the direction of the forces acting upon particles of the beam because of the eld are inward, as shown by arrows l'l. The

effect of the forces due to the magnetic field FLBO where (BP) is the momentum of the beam, Bn the magnetic induction at the outer edge of the conductor, and a. and L are dimensions as defined inv Fig. l.

Referring now to Fig. 3, a cylindrical tube 2l, preferably Vformed of a non-conducting material such as glass, serves as an envelope. Each end 3 of the tube 2l has an outwardly beveled portion 22 to receive a clamping ring 23 having an oppositely beveled opening 24 therein. To provide an electrode 26 at each end 0f the tube 2l, a hollow metallic cylinder 21, preferably made of aluminum, is fitted thereto by means -of an axially extending portion 2`8 having an outside diameter substantially equal to the inside diameter of the tube 2|. An outwardly extending flange 29 is provided on the electrodes ,26 and is utilized for suitably securing each of the electrodes to the respective clamping ring 23 as by bolts 3l. Preferably a sealing gasket 32 of rubber, or vother suitable non-conducting material, is disposed b etween the end of the tube `2| and the flange 29 to ensure an air-tight connection. Each of the electrodes 26 is provided with an aperture 33, one for suitable connection to a source (not shown) of gas, such as hydrogen, and the other for suitable connection to a vacuum pump (not shown). The outermostend of eachr of the electrodes is closed .v by a metallicfoil 36, chosen to be substantially transparent to the beam which it is desired to focus. Such foil 36 is suitably secured to a retaining ring 31 which in turn is fastened to the electrode 26 by means of bolts 38. To provideV an air-tight connection between the ring 31 and the electrode 26, a sealing gasket 39 is disposed therebetween.

One of the electrodes 26 is electrically connected to a ground connection by a lead 5I. The other electrode 26 is connected to one sphere 52a ofv a sphere gap 53. The other sphere 54 of the sphere gap 53 is connected to the ground connection, 50 through a storage condenser 56. To charge the condenser 56 a conventional power supply 51 is connected across the condenser with a currentlimiting resistor 58 between the power supply and the junction between the sphere 54 and'condenser 56.

.A further connection is made from the junction of vthe electrode 26 and the sphere 52 to a parallel circuit, comprising a resistor 6 l and condenser 62, which is connected to the ground connection 56through the secondary winding 63 of .1;

a pulse transformer 64. One side of the primary winding 66 of the transformer 64 is connected to thel ground connection 50 and the other side is connected 'to the anode of a tube 61 through a condenser` 68. Also connected to the anode of the-tube 61 is one side of a current limiting resistor 69, the other side of which is connected to a terminal 1I of a conventional power supply 12.v A terminal 13 of the power supply 12 is cony nected to the ground connection 56. The cathode of the tube 61 is directly`connected to the ground connection 50. The control grid of the tubeY 61 is connected to the ground connection 50 through a resistor 14 and to one terminal 15 of a conventional pulse generator 11 through a couplingcondenser 18. The other terminal 19 of the generator 11 is connected to the ground connection 56.

` Consider now the operation of the present invention with the current carrying conductor assembled and the electrical connections thereto accomplished in the foregoing described manner. First, the vacuum pump is used to exhaust the air from the current carrying conductor and then hydrogen is introduced to a pressure of .3 to .5 mm. as` a suitable medium for an arc discharge.

The condenser 56 is charged by the power supply .51; however, the Value of the voltage is insufficient to cause a break down inthe gap between. the spheres 52 andr 54. Because of the foregoing, and to control the discharge of the condenser 56, it is necessary to apply a trigger voltage and this is accomplished by the remainder of the electrical circuit shown on Fig. 3. With the occurrence of a positive pulse of voltage at the output of the generator 11, the tube 61 conducts and discharges the condenser 68, which has been previously charged by the power supply 12. The discharge of the condenser 68 results in a surge of current through the primary winding 66 of the pulse transformer 64. Since the pulse transformer 64 is a step-up transformer, the secondary winding 63 develops a high value voltage pulse which is applied across the sphere gap 53 and the condenser 56 through the condenser 62 and resistor 6I combination. Such a pulse of voltage causes a breakdown in the gap between the spheres 52 and 54 and results in the condenser 56 discharging through an arc established between the electrodes 26.

With the arc established, it will be readily apparent that a magnetic field, similar to that illusv trated in Figs. l and 2, is established. Therefore a beam of high energy particles directed through the arc in the same direction will undergo forces which focus such beam.

It has been found that a synchro-cyclotron proton beam having an energy -as high as 350 Mev may be easily and eifectively focused with the present invention. The retaining ring 31, to which the sphere gap 53 is connected, was sealed over an aperture in the vacuum tank of `the synchro-cyclotron at the point of beam emergence. The entrance foil 36 was -made of lx/z mil dural foil and was found to produce no appreciable broadening of the beam by multiple scattering in the foil, even at the higher energies. It was possible to use such a thin entrance foil because of the slight variation in pressure on either side. As stated previously hydrogen ata pressure of .3 to .5 mm. is a suitable medium for the arc discharge and was maintained during operation. It was found advisable to circulate the hydrogen continuously to purify the gas from gases generated from the surfaces of the elec-V trodes. The exit foil 36 at the other retaining ring was made of somewhat thicker material, approximately .010 in..of aluminum, to 4withstand the difference in pressure involved and any scattering effect may be minimized by placing the target close thereto.

The arc was fed from the condenser 56 through* the sphere gap 53 triggered by a 70 kv. pulse transformer. The arc was operated at a pulse repetition rate of approximately 60 per second,v

in synchronism with the` cycleV of the synchrocyclotron, by initiating operation of the pulse generator 11 each time the electrostatic deector of the cyclotron was operated to bring the beam out. Under these conditions the arc operated with a current of 4060 amperes and an arc drop of9 kv. This magnetic arc lens installation, described as an illustration, focused thediverging beam into a well defined beam having a considl erably smaller cross section and a much higherV density. Y k K From the foregoing -it will be readily apparent that such an arrangement could be easily utilized to bend a beam of high energy particles by forming-v an, arc discharge through a tube which is bent in any predetermined manner. I n such instance the forces of the magneticeld established by the arc would force the beam to follow the arc. While the salient features of the presentinvention havebeen described with'respect-to one embodiment it will, of course, be apparent that numerous modications and changes may be made within the spirit and scope of the invention and it is therefore not desired to limit the invention to the exact details shown except insofar as they may be defined in the following claims.

What is claimed is:

l. A magnetic arc lens adapted to be disposed in the path of and parallel to a beam of high energy particles, comprising an envelope dening a chamber, a pair of tubular electrodes disposed one at each end of said envelope, sealing means substantially transparent to said beam mounted across the outer opening of said electrodes, means for maintaining a gas medium in said chamber, and electrical means connected to said electrodes for establishing an arc between said electrodes in the direction of said beam whereby the magnetic eld of said arc exerts a focusing force upon particles of said beam.

2. A magnetic are lens adapted to be disposed in the path of and parallel to a beam of high energy p-articles, comprising a tubular envelope dening a chamber, a pair of tubular electrodes having a cross section substantially the same as said envelope and being disposed one at each end of said envelope, sealing means substantially transparent to said beam mounted across the outer opening of said electrodes, means for evacuating said chamber, means for introducing a gas medium into said evacuated chamber, and electrical means connected to said electrodes for establishing an arc therebetween in the direction of travel of said beam whereby the magnetic field of said arc exerts a focusing force upon particles of said beam.

3. A magnetic arc lens adapted to be disposed in the path of and parallel to a beam of high energy particles, comprising an elongated glass tube dening a chamber, a pair of metallic tubular electrodes having a cross section substantially the same as said envelope and being mounted one at each end of said envelope, metallic foils substantially transparent to said beams mounted across the outer opening of said electrodes, means for evacuating said chamber, means for introducing a gas medium into said evacuated chamber, and electrical means connected to said electrodes for establishing an arc therebetween in the direction of travel of. said beam whereby the magnetic eld of said arc exerts a focusing force upon particles of said beam.

4. A magnetic anc lens adapted to be disposed in the path of and parallel to a beam of high energy particles, comprising an envelope defining a chamber, a pair of tubular electrodes disposed one at each end of said envelope, sealing means substantially transparent to said beam mounted over the outer opening ci? said electrodes, means for maintaining a gas medium in said chamber, a sphere gap and a condenser serially connected between one of said electrodes and a. ground connection, the other of said electrodes being connected to said ground connection, means connected across said condenser for charging said condenser to a voltage less than the breakdown voltage of said sphere gap and the gap between said electrodes, and means connected across said sphere gap and condenser for applying a pulse of voltage to cause a breakdown of said sphere gap and discharge said condenser through an arc established in the direction of said beam between said electrodes, whereby the magnetic iield of said are exerts a focusing force upon particles of said beam.

5. A magnetic arc lens adapted to be disposed in the path of and parallel to a beam of high energy particles, `comprising an envelope defining a chamber, a pair of tubular electrodes disposed one at each end of said envelope, sealing means substantially transparent to said beam mounted over the outer opening of said electrodes, means for maintaining a gas medium in said chamber, a sphere gap connected to one of said electrodes, the other of said electrodes being connected to a ground connection, a storage condenser connected between said sphere gap and said ground connection, a power supply connected across said storage condenser for charging said condenser to a voltage less than the breakdown voltage of said sphere gap and the gap between said electrodes, the secondary winding ci a pulse transformer connected across said sphere gap and storage condenser through a coupling condenser, and means connected to the primary winding of said pulse transformer for applying a pulse of voltage thereacross, whereby an arc is established between said electrodes to develop a magnetic eld having a focusing action on particles of said beam.

6. In. a device for focusing charged particles, the combination comprising a tubular discharge chamber having a rst particle transparent electrode and a similar second electrode spaced therefrom, said first electrode being connected to ground, a spark gap having one side connected to said second electrode, a condenser connected between the other side of said spark gap and ground, means for charging said condenser, and connected to said spark gap means for triggering said spark gap whereby said capacitor is discharged through said tubular discharge chamber.

WOLFGANG K. H. PANOFSKY. WILLIAM R. BAKER.

REFERENCES CITED The following references are of record in the le of this patent:

A Focusing Device for the External 350 Mev Proton Beam of 184 inch cyclotron at Berkeley, by W. K. H. Panofsky and W. R. Baker, The Review of Scientific Instrument, vol. 21, No. 5, pp. 445-447, May 1950. 

